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Testa
Account closed
Registered: Oct 2004
Posts: 197 |
read $dc0d
when i set up a raster irq i always do lda #$7f sta $dc0d lda $dc0d.. no problem but there is one thing i dont get:
at $ea7e there is also a lda $dc0d.. can i savely skip the lda $dc0d in the irq set up and end the irq with a jmp $ea7e... or is there a diffirence??? |
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Testa
Account closed
Registered: Oct 2004
Posts: 197 |
ok boys thanks for the info... very nice.. |
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CreaMD
Registered: Dec 2001
Posts: 3057 |
When I'm stumbling over debates like this I wonder if god gives me the signal that c64 scene is still kicking some major ass. I'm very thankful for that. |
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HCL
Registered: Feb 2003
Posts: 728 |
Quote: JackAsser: You cycle waster; jump to $dc0c after setting it to $40! ;D
Hmm, that one beats me.. or did you mean setting it to $4c?! $40 is RTI on my hardware :).
Ok, thanx guys! Yeah, it must have been the BRK/IRQ i had in mind somewhere..
So.. in this case it seems like ASL $d019 and LSR $d019 gives the same result!?
TFM says ($d019):
Bit 7: 1 = IRQ has been generated.
Bit 0: Raster Compare IRQ Flag (see $D012).
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JackAsser
Registered: Jun 2002
Posts: 2014 |
Quote: Hmm, that one beats me.. or did you mean setting it to $4c?! $40 is RTI on my hardware :).
Ok, thanx guys! Yeah, it must have been the BRK/IRQ i had in mind somewhere..
So.. in this case it seems like ASL $d019 and LSR $d019 gives the same result!?
TFM says ($d019):
Bit 7: 1 = IRQ has been generated.
Bit 0: Raster Compare IRQ Flag (see $D012).
Ofcourse ASL and LSR is the same since either bit 7 or bit 0 can be used to check the state of the raster IRQ.
A software IRQ generated by a BRK opcode can indeed be polled by fetching the cpu status word using the PLP opcode.
Placing #$40 (RTI) on $dc0c is useful since the 6502 always fetches two consecutive bytes (and dismiss the last one in case of NOP, RTI etc.). Hence the CPU will actually read $dc0d (ack the timer irq) then you jump to $dc0c.
jmp $dc0c; rti = 3c+rti
bit $dc0c; rti = 4c+rti
One cycle saved!
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Oswald
Registered: Apr 2002
Posts: 5094 |
rti always reads the byte behind the rti aswell, so it will ack the interrupt. |
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Oswald
Registered: Apr 2002
Posts: 5094 |
Quote: When I'm stumbling over debates like this I wonder if god gives me the signal that c64 scene is still kicking some major ass. I'm very thankful for that.
no it does not. |
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HCL
Registered: Feb 2003
Posts: 728 |
Quote: no it does not.
Maybe you don't, bet we do ;). Weird shit in this thread, possibly i learnt something new, which is also totally unusable :). Thanx guys! |
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